Typical EUV sources for lithography and inspection tools are based on plasma formation which result in radiation emission over a broad spectral range from about 5 nm to the infrared (IR). EUV mirrors based on Molybdenum and Silicon multilayers reflect not only EUV, but also radiation above about 100 nm. This so called “out-of-band” radiation can lead to reduced resolution or increased noise floor on the detector. For these reasons, it is often desirable to reduce or remove the “out-of-band” radiation from the useable EUV radiation before the useable radiation reaches the sample or the detector. In addition, the presence of “out-of-band” vacuum ultraviolet (VUV) and ultraviolet (UV) radiation can lead to increased oxidation as well as carbon and inorganic contamination growth on exposed surfaces. This oxidation and contamination can limit the lifetime of components such as mirrors and detectors.
For the reasons cited above, spectral purity filters based on thin metal films are often used to suppress unwanted radiation. These thin metal films usually have thicknesses in the range between 8-200 nm and can have apertures of several centimeters in diameter. Conventional spectral purity filters based on thin films are either free-standing or supported by a mesh.
Freestanding SPF filters usually have thicknesses in the 100-200 nm range. As a result, they are extremely fragile and can be sensitive to gas flows and pressure differences. For some EUV tool designs, gas flows are used for suppressing contaminant movement and particle protection. Also, for these EUV tools, pressure differences can be generated during initial vacuum pump down. In some cases, these gas flows and pressure differences can lead to a failure of the fragile SPF filter. A failure here can lead to an immediate, complete destruction of the filter, generating particles that can spread and contaminate the tool environment.
Heretofore, supported filters have typically been made by attaching a thin SPF film to a relatively thin (usually Ni) mesh. In order to maximize transmission through the SPF filter, a conventional mesh typically has a large ratio of open area to area blocked by the mesh structure, such as 1 mm openings and wire diameters in the 10th of micrometer scale. The supported filters generally use special adhesives for affixing the mesh to a frame. However, these adhesives can outgas in vacuum environments. This can increase the contaminant level inside an EUV tool, reducing the optics and detectors, for example, due to carbon contamination.
Generally, tools that use EUV light operate in an ultraclean vacuum environment and in the presence of gasses, such as H2, Argon and/or other possible gas species. In this environment, acceptable material choices are very restricted. Only certain types of metals and very few types of other materials qualify to be low outgassing with qualified clean procedures. In addition, the chosen materials may need to withstand exposure to EUV radiation and withstand exposure to reactive Argon, H2O and hydroxides and hydrogen species (neg and pos Hydrogen and atomic Hydrogen). For example, the use of cables or adhesives can lead to contaminants (mainly Hydrocarbons in the gas phase) inside the chamber and can lead to particulation. It is also known that the interaction of EUV light with contaminants can generate layers of contaminants (e.g. Carbon) on surfaces. This effect is more pronounced on surfaces that are actually exposed to EUV radiation, e.g. mirrors. Here, contamination can cause a reflectivity drop and can cause a phase change of the incident radiation. Both of these effects can degrade the optics over time, and in some cases lead to a failure of the optical system.
With the above in mind, Applicants disclose Spectral Purity Filters (SPF), Support Structures for SPF Films and Corresponding Methods for Manufacture.